Fused tungsten carbide (FTC) is a particularly advantageous compound for wearing protection because of its high hardness and toughness. However, the use of FTC in dispersions for coating purposes and for producing drill bits is limited by the heat treatment required. In the heat treatment, the FTC is exposed to a liquid metal bath and parts of the FTC dissolve in the metal bath, which later has an adverse effect on the properties of the treated component. The temperature employed in the production process is a function of the binder metal used. The dissolution rate of the FTC in turn depends on the chemical nature of the binder metal used itself and on the temperatures employed. The degree to which customary binder metals attack FTC decreases in the order Fe>Ni>Cu. A hard material having a shell composed of tungsten carbide and a core composed of fused tungsten carbide combines the high hardness and toughness of the fused carbide in the interior of the particle with the chemical resistance of the WC shell in a metallic matrix at high temperature. This particular hard material will hereinafter be referred to as macroline FTC.
A particularly advantageous use form of hard materials is a dispersion in an organic binder. A dispersion can be produced and, for example, applied to a surface simply and inexpensively. Targeted setting of the viscosity, the addition of further hard materials or metal powders or further mechanical upgrading steps such as kneading or calendering enable dispersions to be produced for particular applications. However, the setting of the viscosity influences the processing properties of the dispersions. A particularly advantageous range for the processing of hard material dispersions is in the viscosity range from paints and pastes through to self-supporting but flexible components such as foils (sheets).
Fields of use of the dispersions are, in particular, in the field of surface coating but also in the production of drill bits. A dispersion is here advantageously joined by soldering processes or infiltration to the component subject to stress. The solder material necessary for metallurgical bonding is frequently likewise made available as a dispersion, e.g., as a tape. The hard material dispersion and the solder dispersion can also be present as multilayers. As an alternative, the solder material can also be supplied independently of the hard material, as in, for example, the infiltration of a drill bit.
DE 10 354 543 B3 describes the production of a tungsten carbide powder (cFTC), with the powder particles having a core composed of fused tungsten carbide (FTC) and a shell composed of tungsten monocarbide. The production of cFTC is carried out by heating a fused tungsten carbide powder in the presence of a carbon source to a temperature of 1300-2000° C. The cFTC powder can have a broken, sharp-edged or spherical morphology. The great advantage of cFTC is the chemical resistance to liquid metals, which is of great importance in, for example, the soldering-in and welding-on of coatings or in the infiltration of drill bits. The stabilization of the FTC by the WC shell against the binder metals, usually nickel, copper, cobalt or iron alloys with various alloying elements such as Mn, P, B or Si, leads to substantial retention of the hard material content used and thus to avoidance of the precipitation of brittle carbides during the solidification operation and to stabilization of the viscosity of the metal bath. DE 10354543 B3 claims, inter alia, the use of FTC for surface coating by means of conventional methods proceeding from powders or powder mixtures, e.g., welding or spraying.
DE 2 115 358 and U.S. Pat. No. 3,743,556 describe a process for producing layers composed of a fine filler dispersed in a metal. The production of tapes containing organic materials, metal powder and tungsten carbide has, according to U.S. Pat. No. 3,743,556, been prior art since the 1970s. The use of diamonds as a filler is also mentioned therein. However, the abovementioned documents do not indicate how the diamonds can be placed at defined positions in the tape.
U.S. Pat. No. 5,164,247 describes a method of improving the wearing resistance of layers produced by means of filled tapes. Densificiation of the tape by means of rolling is here proposed.
U.S. Pat. No. 6,649,682 B1 describes the use of hard materials which are applied like a paint for wearing protection, in particular, for producing particularly thin layers. It is expressly emphasized that the hard materials used must not dissolve in the metal bath.
U.S. Pat. No. 6,309,761 B1 describes a general problem of the temperature sensitivity of the materials used in coating by means of soldering processes, as are preferably used in, for example, the application of tapes, pastes or paints to steel bodies. The heat input, e.g., during the soldering process, has an adverse effect not only on the hard material but also on the substrate. The problem is circumvented by using a “perform”. This means that the coating is applied to a thermally insensitive metallic substrate and then applied together with this metallic substrate to the actual component to be coated. This solution is a variant for the protection of the microstructure of the component to be coated.
WO2004/085690 describes a method of producing spherical fused tungsten carbide (FTC) and monophase tungsten carbide (WC) with the addition of doping elements selected from the group consisting of Ti, V, Nb and Ta.
In all the inventions cited, one of the best materials existing in the sector of wearing protection, classical fused tungsten carbide, is not used in practice for coating by means of soldering operations. The reason for this is the dissolution of FTC in the metal bath during the soldering operation, which makes the protective layer unusable.